1. Field of the Invention
The present invention relates to liquid crystal alignment, and more particularly to an apparatus and method for preparing surfaces to provide tilted and vertical alignment of liquid crystal material.
2. Description of the Related Art
Liquid crystal (LC) material employed in liquid crystal displays typically relies on alignment layers to establish a stable pretilt angle for the liquid crystal material. Typically, the alignment of the liquid crystals for flat panel liquid crystal displays (LCD) is accomplished by placing a thin film of LC material on a mechanically rubbed polyimide film coated on a suitable substrate. Limitations imposed by the mechanical rubbing method (e.g., creating multiple domains for improving the viewing angle) in conjunction with the difficulty of optimizing polymer materials (e.g., polymers that avoid image sticking) make it highly desirable to use alternative materials and a non-contact LC alignment method.
There are a number of different methods/materials which have been shown to create LC alignment besides rubbing. For example, these methods/materials may include a stretched polymer, a Langmuir Blodgett film, a grating structure produced by microlithography, oblique angle deposition of silicon oxide, and polarized ultraviolet (UV) irradiation of a polymer film.
Non-contact methods to replace rubbing are described in commonly assigned U.S. Pat. No. 5,770,826, incorporated herein by reference, which describes a particularly attractive and versatile LC alignment process based on ion beam irradiation of a polyimide surface. The method places the LCs on a polyimide surface which has been bombarded with low energy (about 100 eV) Ar+ ions. This process has many characteristics which make it suitable for the manufacture of LC displays. This method has been extended to include diamond-like carbon (DLC), amorphous hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al2O3, CeO2, SnO2, and ZnTiO2 films as described in commonly assigned U.S. Pat. No. 6,020,946, incorporated herein by reference. Another method for creating an LC alignment layer in a single deposition process has been described in commonly assigned U.S. Pat. No. 6,061,114, incorporated herein by reference.
Ion-beam treatment on DLC films (IB/DLC) for the alignment of liquid crystals has many advantages over conventional rubbed polyimide alignment, such as, non-contact processing, alignment uniformity, etc. Usually, DLC films of about 50 angstroms thick are deposited by plasma enhanced chemical vapor deposition (PECVD), and followed by Ar ion beam irradiation. It is believed that the Ar ion beam destroys the amorphous-carbon rings which have a large collision cross section to the ion beam. The amorphous-carbon rings which have a small or zero collision cross section to the ion beam are preserved. The average direction of the remaining carbon rings align the liquid crystal and generate a pretilt angle. The pretilt angle of IB/DLC alignment is not stable. The pretilt angle tends to decrease when the IB/DLC substrates are in contact with moisture or other components in air. The pretilt angle decreases as a function of storage time in vacuum-sealed LC cells with IB/DLC alignment. In addition, the pretilt angle is not stable under ultra-violet (UV) or violet irradiation.
After ion-beam treatment, the surface of the DLC films are very active due to the ion-beam induced free radicals on the DLC surface. These free radicals tend to react with many chemical species in contact with them. This reaction may change the surface chemistry of the DLC film or change the orientation the carbon rings. As a result, the pretilt angle will degrade.
Vertical alignment liquid crystal displays (LCD) have become one of the top candidates for LCD monitor applications. This is due, in part, to the wide viewing angle and fast response time provided by vertical alignment LCDs. To control declination in the vertical alignment LCD, a certain pretilt angle is needed. However, good vertical alignment, especially with a pretilt angle, is very difficult to obtain.
Conventional alignment methods include surfactance attachments on oblique evaporated silicon oxide surfaces and rubbed side chain polyimides, as described above. The surfactance attachments approach requires double evaporation processing plus a surfactance attachment. This approach is very complicated and the charge holding ratio of the resultant surface is often excessive. The rubbed side chain polyimides approach uses rubbing to generate pretilt angle. This process is difficult to control and leaves rubbing traces on the display.
Using single oblique evaporation of silicon dioxide to obtain vertical and tilted vertical alignment, high quality vertical alignment may be provided. Van der Waals interaction has been used to explain this phenomena. However, the deposition may be slow and may include unconventional semiconductor processes. The vertical alignment of negative dielectric anisotropic liquid crystal (LC) on silicon dioxide is dominated by the van der Waals interaction and not due to the surface morphology. The thin film deposition method is therefore not limited to oblique evaporation. Other thin film deposition methods, such as sputtering, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD), can be used to produce zero tilt vertical alignment, as well.
However, since the directionality of sputtering, CVD and PECVD are not as good as oblique evaporation, it is difficult to obtain a suitable pretilt angle. For example, sputtered silicon dioxide from a tilted target can give a pretilt angle of 0.4 degrees. This may be enough to control the domain formation, but the response time is slow.
Therefore, a need exists for a reliable generalized method to produce vertical and tilted vertical alignment by a thin film deposition process and ion beam treatment.